Conformations of $n$-butane

Rotation about the central CC bond in $n$-butane gives different conformations:

• Anti: methyl groups are $180^\circ$ apart  lowest energy
• Gauche: methyl groups are $60^\circ$ apart  slightly higher energy
• Eclipsed (CH$_3$-H eclipsing): higher energy
• Totally eclipsed (CH$_3$-CH$_3$ eclipsing): highest energy

General increasing order of potential energy

Thus, for $n$-butane:

Matching with the given labels

From the standard labeling used in such questions:

• $I$ = anti
• $III$ = gauche
• $IV$ = eclipsed
• $II$ = totally eclipsed

Therefore, increasing potential energy is:

Compare with options

This matches Option D.

Comparison with stored answer

Stored correct answer = D

Our derived answer = D

So they agree.